Unsaturated aliphatic hydrocarbons are the building blocks of the organic chemical industry. The term “unsaturated” indicates that the carbon atoms do not have the maximum number of hydrogen atoms attached to them. Four bonds are possible for each carbon atom. Saturated hydrocarbons bond with four other atoms (a combination of carbon and hydrogen atoms). Unsaturated hydrocarbons contain at least one carbon-carbon multiple bond. A double bond leaves room for only two other bonds with hydrogen or carbon. A triple bond allows only one other bond with hydrogen or carbon.
Compounds with double bonds are also commonly known as alkenes or olefins. By far the most important alkene is ethene (ethylene), followed by propene (propylene), and butadiene. Ethyne (acetylene) is the most important alkyne, as compounds with a triple carbon-carbon bond are called.
Compounds with two or more multiple bonds are termed polyunsaturated (a characteristic of many natural fats and oils). Where there are two double bonds in an alkene, they may be differently arranged. The color of many organic compounds depends on the arrangement of double bonds in the molecule.
Multiple bonds are generally more reactive than single carbon-carbon bonds. Compounds containing multiple bonds may undergo polymerization, cyclization, and addition reactions. Polymerization is economically the most important of these. Flydrocarbon polymers (large, chainlike molecules) include plastics, resins, and both natural and synthetic rubber. In a cyclization reaction, a straight chain curls around to form a ring. Several important solvents are made in this way. Addition reactions break one of the bonds in a multiple bond and add two new groups to the molecule, one at each end of the broken bond. It is this reactivity of the multiple bond, as well as the many synthetic possibilities, that makes these compounds so important in nature and in the chemical industry.
One result of multiple bonding is that the two linked carbon atoms are not able to rotate freely, as they would if joined by a single bond. If an alkene has two different atoms or groups attached to each carbon atom, it can exist in two physically different forms, depending on whether the groups are on the same side of the molecule or on opposite sides.
Uses of alkenes
Simple alkenes are produced from natural gas and also from naphtha, a straw-colored liquid obtained by refining crude oil. The process used to break the large molecules of crude oil molecules down into smaller ones is called “cracking.” Naphtha, a mixture of saturated hydrocarbons, undergoes thermal cracking at 1000°-! 200° F. (540°- 650° C) in the absence of air, breaking down into unsaturated hydrocarbons such as ethene, propene, and butadiene.
Ethene, a colorless, highly inflammable gas with a sweet odor and taste, is the single most important organic chemical feedstock. It is also an important natural substance that helps fruit to ripen. About half of the ethene (ethylene) produced industrially is used to make the plastic polyethylene. Many other plastics, synthetic rubbers, and resins can also be derived from it. Ethene is used in making antifreeze, cosmetics, paints, lacquers, and acetic acid, the key ingredient of vinegar.
Propene (propylene) is a colorless, inflammable gas that can be used to produce isopropyl alcohol, from which acetone is made. Acetone is an important organic solvent. Cumene, produced from propene and benzene, is used to make phenol. Phenol (carbolic acid) is used in making antiseptics and disinfectants. Propylene glycol (another alkene product) is a solvent for fats, oils, resins, perfumes, colors and dyes, soft-drink syrups, and flavor extracts. Butadiene is important in the manufacture of synthetic rubber and certain types of resins. These substances are extremely hard, durable, and resistant to fire.
Higher alkenes can be produced by thermal cracking of waxes, which are saturated hydrocarbons, or by building up a long chain from ethene. These alkenes are used to make detergents and lubricants.
Acetylene and the alkynes
Ethyne (acetylene) is a colorless, highly flammable toxic gas, used for welding because it burns with an intense, hot flame. Carbon black—fine particles of carbon produced by burning ethyne in a limited supply of air—is used as a pigment as well as in tires and plastics. Ethyne is produced mainly by the cracking of oil but can also be obtained by reacting calcium carbide and water.